This invention relates to a fluidized bed system and a nozzle for use therein and, more particularly, to such a system having a bed of particulate material in an enclosed space which is fluidized and cooled by the introduction of air and water into the bed through a plurality of nozzles.
Fluidized bed reactors, such as gasifiers, steam generators, combustors, and the like, are well known. In these arrangements, pressurized air or other fluidizing media is passed, via a plurality of nozzles, through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. The entrained particulate solids are usually separated externally of the bed and recycled back into the bed. The heat produced by the fluidized bed is utilized in various applications such as the generation of steam, which results in an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
However, fluidized bed reactors require careful control of several parameters, including particle size distribution to avoid increasing the volume of the relatively fine particles in the reactor to unacceptably high levels. Also, bed temperature must be controlled in order to prevent undesirable temperature excursions and agglomeration of the bed material.
Particle size distribution is often controlled by removing the relative fine particles from the reactor, stripping them with a stream of air in a secondary fluidized bed and recirculating them back to the reactor. An example of this technique is disclosed in U.S. Pat. No. 4,829,912, assigned to the assignee of the present invention. These secondary fluidized beds can also perform a cooling function and are often termed "stripper/coolers". A stripper/cooler located adjacent the furnace section of the reactor can accomplish both the recirculation of the finer portions of the removed, particulate material and the removal of heat from the removed but non-recirculated, particulate material. In these types of arrangements, the stripper/cooler receives the particulate material from the furnace section through a drain pipe and air is blown through a first section of the stripper/cooler to strip, or entrain, some of the finer portions of the particulate material which are returned to the furnace section. The remaining particulate material in the stripper/cooler is then usually passed to a cooler section where heat is removed from the particulate material by passing water/steam in a heat exchange relation to the particulate material or by blowing air through it before it is discharged from the system. When air is used to remove the heat from the non-recirculated particulate material, this air is often returned to the furnace section as preheated combustion supporting air.
However, in some situations, such as when fuels that generate an excessive amount of relatively fine ash are used, or when a relatively large amount of relatively fine adsorbent has to be used with fuels having a relatively high sulfur content, the relatively fine particle material stripped in the stripper/cooler and returned to the furnace section increases the volume of the fines in the furnace section, or the upper furnace loading, to unacceptably high levels. Excessive upper furnace loading requires larger and more expensive stripper/coolers and separators and/or requires that the furnace be operated at a low stoichiometric condition, which is inefficient.
This upper furnace loading is made worse when the method used to cool the particulate material in the cooler section of the stripper/cooler is by blowing air through the material. To achieve a high cooling rate, the air velocity and flow rate through the cooler section must be relatively high. A high air velocity and flow rate, however, entrains greater amounts of particulate material resulting in an even greater volume of fines returned to the furnace section when this air is used as combustion supporting air, thereby further increasing the upper furnace loading. To complicate the matter even further, a high air velocity in the cooler section is also necessary to prevent agglomeration of the particulate material in the stripper/cooler caused by relatively high temperatures in the stripper/cooler due to the combustion of unburned carbon. Making the stripper/cooler larger in area to alleviate this concern does not help since unacceptably high amount of combustion air is used in the stripper/cooler, thereby leaving less than adequate air for proper process control requirements.
Thus a significant need arises for improved temperature control of a fluidized bed reactor and stripper/cooler.